2015 May 7

Lecture in middle of first op-amp lab

The lecture between the halves of the first op-amp lab did not cover much material. A big chunk of the first part was a discussion with the class about whether we should have a midterm quiz. After much discussion of the advantages and disadvantages of different approaches, we finally decided that I would give them a take-home, ungraded quiz, so that they could test themselves and later ask questions in class for things they needed more help with. This discussion also brought out some suggestions from students of additional resources that they had found useful (Khan Academy videos, the new edition of Horowitz and Hill, and the All About Circuits web textbook). I also got a chance to give them some reassurance that they are doing well, since some are getting discouraged. I’m packing a lot into the class, and it is easy for the students to get overwhelmed—especially since some are just now getting to capacitors in their algebra-based physics classes.

Because most of the class had working audio amps in the Tuesday lab, I made a number of suggestions for a tinkering lab on Thursday. In addition to the ones I already mentioned in

Switching from a symmetric dual power supply to a single power supply.

Paralleling two op-amp chips to get twice the current capability.

Adding a potentiometer for variable gain.

Adding a unity-gain buffer to separate the loudspeaker driver from the gain amplifier.

Adding a tone-control circuit, like the Baxandall tone control on http://www.learnabout-electronics.org/Amplifiers/amplifiers42. They can’t use exactly that circuit, as they have only 10kΩ potentiometers, not 100kΩ ones. The idea can be adapted, or the students could do simple treble-cut or bass-cut circuits.

Using a loudspeaker as a microphone. I think that should work, as I get about a 500µV signal from my loudspeaker when I talk into it. The don’t need any DC bias for the loudspeaker mic, and they may even be able to eliminate their high-pass filter, as the loudspeaker mic can be set up to have its output already centered at 0V.

I also suggested hooking up a plug to the output of a music device, investigating how the amplifier clips, and hooking up a function generator (with a voltage divider) to replace the input and high-pass filter so that gain can be measured without the difficulty of varying sound level in the room. The point of the lab (after they’ve done a good job of explaining how they designed the basic amplifier) is to get them to play with the design—to do something they see as fun.

I also talked about why some student had been seeing asymmetric clipping when they hooked up their loudspeakers. The key concept (which they had not had) is that the input-referenced voltage offset could be as large as ±4.5mV for the MCP6004 op amps that they are using. With a gain of 50, that makes an output offset of up to ±225mV, but with an 8Ω loudspeaker the current limits cause clipping at about 200mV, so the output signal could be shifted far enough so that half of it is clipped, even it all looks like it should be in range. I talked a little about the possibility of doing offset nulling, but didn’t really give them a circuit that they could use.

In lab today, people did seem to be having a lot of fun, and both morning and afternoon sessions ended early. I’m looking forward to reading the design reports this weekend, because they should be different in interesting ways, as different students chose different directions to explore. I helped a few students debug their circuits (as usual, the most common problems were loose wires, power supply not providing power, and scope probes set differently from what the oscilloscope thought).